Cathodic Behaviour of Dicationic Imidazolium Bromides: The Role of the Spacer

The cathodic reduction of dicationic imidazolium bromides,whose spacer is either an aliphatic chain or a xylyl group, leadsto the formation of the corresponding N-heterocyclic carbenes(NHCs), which were isolated as the corresponding thiones, afterreaction with elemental sulfur. The behaviour of the dicationswas compared with the corresponding monocations. Thebehaviour of dicarbenes depends on the nature of the spacer.This study evidenced that dicarbenes deriving from xylyldications are less stable than the corresponding aliphatic ones(giving lower yields in thiones), due to a debenzylation reaction.On the other hand, the yields in thiones starting from aliphaticdications are higher than the corresponding monocations,suggesting a cooperative reduction at the electrode of the twoimidazolium moieties. The cathodic process was confirmedusing the co-electrogenerated hydrogen to reduce 2,2,2-trifluoroacetophenone to the corresponding alcohol.